Rocket with internal rod



sa@ i 62 A' T' CAMP ROCKET WITH INTERNAL ROD Filed April 21, 1955 INVENTOR. i@

ALBERT T. CAMP m xt SW ATTORNEYS States The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to an improved jet actuated device incorporating a new ballistic modifier composition for double base propellants and to the method of external introduction of the composition into double base propellants.

The description and advantages of mesa and plateau type propellant compositions for use with jet actuated devices are set forth in my classified application Serial No. 352,312 for Gas Producing Charge filed April 30, 1953 and allowed February 17, 1955. As is Well known, the percentage composition ranges of ballistic modifiers and other ingredients in the above type compositions Which impart to them the plateau and mesa characteristics are quite critical, Accordingly, these cha-racteristics are severely affected by minor changes in composition.

A major advantage of the above type propellants is their temperature insensitivity within Well defined ranges. However, the propellants are subject to further improvement in this respect in that the range within which they are temperature insensitive does not include the full range of service temperatures required for some applications of jet actuated devices, such as those of rockets wherein operations within a range from -65 F. to +165 F. are required.

A disadvantage attendant to the use of mesa and plateau type propellants in jet actuated devices, a-nd particularly in aircraft rockets, is the excessive secondary flame produced by the burning of the exhaust gases, consisting largely of hydrogen and carbon monoxide. This secondary burning results in mechanical difficulties in operation of the launching plane, and damage to launching devices and sections of the plane fuselage and wings.

The beneficial effect of certain potassium salts and other additives on the combustion of conventional single and double base propellants is well known. However, these additives cannot be introduced directly into the mesa and plateau type compositions because of the vitiating effect of even minor additions on the mesa and plateau characteristics of the propellants, and because of the severe undesirable effect their presence produces on the storage stability of the propellant.

It is therefore an object of this invention to provide a jet actuated device which is insensitive in operation to changes in temperature over a wide range.

It is another object of this invention to provide a jet actuated device which is not subject to excessive secondary fiame in operation.

It is still another object of this invention to provide a ballistic modifier for plateau and mesa type propellants to be used in jet actuated devices which widens the temperature insensitive range of these devices, prevents excessive secondary fiame on burning of the propellant, and the application of which is not a factor in storage stability and does not undesirably affect the mesa and plateau characteristics of the propellant.

It is a further object of this invention to provide a means for the external application of ballistic modifiers to propellants as they burn.

It has been found that the above and other objects rasante Patented Dec. i8, MESZ are accomplished by introducing the ballistic modifier externally into the flame area of the propellant as it burns, preferably by means of an incombustible rod or other type holder which has a ballistic modifier incorporated thereon by means of a plastic binder. In the preferred application the ballistic modifier is potassium sulfate and it is incorporated on the outer surface of the rod by means of a plastic composition comprising ethyl cellulose, dibutyl phthalate, butyl methacrylate, polyglycol dimethacrylate and benzoyl peroxide, or equivalents thereof. A further modification in certain applications is the addition of an oxidizing salt such as potassium perchlorate or potassium nitrate as a part of the ballistic modifier.

The invention is best explained by reference to the following description and the accompanying drawing hereby made a part of this application and in which:

FIG. 1 is an elevational view of a coated rod of the invention broken away near the center section;

FIG. 2 is an end view of the coated rod taken in the direction indicated by the arrow;

FIG. 3 is a cross-section of the coated rod taken on line 3--3 of FIG. l;

FIG. 4 is a cross-section of a propellant grain with the coated rod mounted internally thereof, and

FIG. 5 is a partially schematic showing of the assembly of coated rod and rocket grain in a rocket motor.

Referring to FIG. 1, there is shown a supporting rod 10 of incombustible material, such as metal, coated with the ballistic modifier composition lll, to be described later. In FIG. 2 there is shown an end section of the rod bent into the form of a ring l0 for mounting in a groove within the head closure of a rocket to support the rod in the approximate center of the opening in the rocket grain. Other means for mounting the rod may be used. For example, in the case of external burning grains other expedients may be necessary to locate the modifier in the flame area of the grain. In FiG. 3, the numeral l1 indicates the ballistic modifier composition molded onto the rod itil in a cruciform design to give efficient burning of the grain. Other designs such as cylindrical and star-shaped moldings may be used. In FIG. 4, the showing of the rod 10 with coating 11 thereon, mounted in the star-shaped hollow of grain l2, illustrates the assembly of the coated rod in the rocket grain. The manner of assembling the rod and grain in a rocket 13 is shown in FIG. 5.

As used in this specification and claims, the expression ballistic modifier is intended to mean a material which enhances the ballistic performance of the jet actuated device in which the propellant is used. The term jet actuated device is to be construed as a device in which at least a part of the propelling force is furnished by the reaction of burning gases which are expanded through one or more nozzles. The term plateau propellant refers to one in which the slope of the curve of a logarithmic plot of its burning rate pressure relationship approaches zero in a favorable range of operating pressures. In the case of a mesa propellant the slope of a corresponding curve attains a negative value in a region of useful operating pressures. By double base propellant is meant one in which the major ingredients are a high explosive such as niitroglycerin, diethylene glycol dinitrate, methyl trimethylolmethane trinitrate orothers and a suitable high energy polymer such as nitrocellulose. By flame area is meant the area within which the heat and flame from the burning prepellent grain will erode the ballistic modifier. The expression internal burning grain refers to one normally constructed to burn from the center outward but does not exclude burning on other surfaces. The word hollow used in reference to the central opening in propellent grains is not restricted to any particular form of opening through the grain but includes various shapes such as an eight point star-shaped opening, a cylindrical opening, and those formed in relatively irregular shapes such as semicircles, curved, truncated segments, and others. By the expression supported externally is meant, only, not incorporated in the propellant. 5

The ballistic modifier compositions of the invention are illustrated by the following examples but the invention is not limited thereby.

EXAMPLE I Component: Weight percent Potassium sulphate 70.00 Ethyl cellulose 14.65

Dibutyl phthalate 5.00 Butyl methacrylate 9.00 n Polyglycol dimethacrylate 1.20 la Benzoyl peroxide 1 .15

Rods having a coating of the above composition 0.30 to 0.40 inch in diameter and 8 to 16 inches in length were tested in a 2."75 Folding Fin Aircraft Rocket with a 2O propellent grain having the following composition which is representative of propellent grain compositions with which the ballistic modifier is operative.

Component: Weight percent Nitrocellulose 50.00 2* Nitroglycerin 34.9 Diethyl phthalate 10.50 Z-nitrodiphenylamine 2.00 Lead 2ethyl hexoate 1.20 Lead salicylate 1.20 Candelilla wax 0.2.0

Propellent, formulations having generally the above composition are disclosed in my above referred to application and will be referred to herein as the N-5 propellant. 35 The observed effects of the above tests at -675 F. were smoother burning, a desirable increase of about 130 p.s.i.

in saddle pressure and a decrease of about 10 percent in burning time. Saddle pressure is the intermediate low pressure in the pressure-time history. Marked smoother 40 burning was observed at 165 F.

The following comparative results indicate the effectiveness of the invention. All firings were made in 2.7 5 Folding Fin Aircraft Rocket using identical size grains of the N-S propellant formulations referred to above. The ballistic modifier composition on the coated rod contained 70 percent potassium sulfate 30 percent of an ethyl collulose plastic composition containing 70 percent of ethyl cellulose, 20 percent of butyl stearate and 10 percent of a highly substituted phenyl ether as a plasticizer. The coating on the rod `was 0.34 inch in `diameter and 18 inches in length.

It will be noted that when the rod coated With the ballistic modifier of the invention is used, the burning rate actually increases slightly with lower temperatures over a wider range of temperature than is the case jwhen the ballistic modifier is not used. The increase inburning rate with lowering of temperature is highly advantageous to compensate for the reduction in thrust of the rocket 7g as the temperature lowers. Y

The operable limits of the components of the ballistic modifier composition are as follows: potassium sulphate -10 to 95%, ethyl cellulose -3 to 50%, dibutyl phthalate -0 to 30%, butyl methacrylate 0 to 20%, polyglycol 75 dimethacrylate -0 to 5%, benzoyl peroxide -0 to 1%, other plasticizers -0 to 30%. yPotassium perchlorate, potassium nitrate, barium nitrate and certain other salts may be used in an amount from about 0 to about 95% in place of this amount of potassium sulfate.

If i-t is desired to suppress flash, particularly during launching of the rocket, a large amount of potassium salt may be introduced into the llame at this time to produce the desired result. This is in contrast Yto prior methods wherein it is introduced directly within the propellant thus making control of the rate of production of potassium salts and ions entirely dependent upon the burning rate of the propellant. As an example of this phase of the invention, if it is found that a combination of potassium sulfate and plastic binder does not erode with sufficient speed, an oxidizing salt such as potassium perchlorate, potassium nitrate or barium nitrate say be used in combination with the potassium sulfate to provide a faster rate of erosion into the flame.

The -ballistic modifier in the above composition is potassium sulphate. Potassium sulphate is the preferred ingredient as it is compatible with propellant ingredients such as nitroglycerin which has a tendency to migrate from the propellant into the coating. The amount of potassium salt which is required to providethe desired functioning of the lrocket motor is preferably of the order of 0.5 to 2 percent of the weight of the propellant.

Potassium is used as the cation of the salt in preference to other alkali metals because it produces a lower flash than t..e others. The alkali nic-tais form oxides, carbonates and hydroxides believed capable of reacting with the free radical elements in rocket exhaust gases which tend to initiate and propagate secondary combustion. Within the rocket motor the beneficial internal ballistic effect is believed to result from the fact that their presence increases the emissivity of the radiating elements of the ame and hence improves heat transfer to the solid propellant, particularly at low temperatures. 1 he solid par'- ticles per se are believed to be responsible for attenuating the resonance effects and standing waves during burning.

The anion of the potassium salt, although not functionally critical, should be one which produces a stable, compatible salt, i.e., Ithe salts should not be strongly acid or strongly basic because of reactions of this type salt with the propellant. Accordingly, neutral salts are preferred. Potassium chloride, for example, has been found to be operable. Y

Other external yballistic modifiers which maybe used are salts of barium such as barium nitrate or other comparatively inert solids which, like potassium salts, remain as solid particles in the flame; for example, titanium has been found effective as an external ballistic modifier. Other suita-ble materials are other salts of alkali and alkaline earth metals. Other suitable oxidizing salts are nitrates and perchlorntes of alkali and alkaline `earth metals.

The plastic binder consist mainly of plasticized ethyl cellulose with or without a cross-linked form of butyl methacrylate which is polymerized during the curing of the molding. The butyl methacrylate monomer serves as a solvent or softening agent for the ethyl cellulose when processing at ambient temperatures is desired. Other solvents which may be used are methyl methacrylate, ethyl methacrylate, ethyl or butyl acrylate, acrylonitrile or other lmonomeric polymerizable solvents. Suitable plasticizer equivalents lfor dibutyl phthalate are butyl stearate, highly substituted phenyl ether, dioctyl phthalate and many others. The polyglycol dimethacrylate serves as a cross-linking monomer for the butyl methacrylate or other monofunctional monomer. This cross-linking agent is bifunctional and serves `to make a three dimensional polymer out of butyl methacrylate and thus to avoid excessive absorptionV of nitroglycerin vapors into the coating. Other suitable cross-linking agents are diallyl maleate, diallyl phthalate, bis-allyl diglycol carbonate and other compatible bifunctional monomers.

The benzoyl peroxide serves as an initiator for the polymerization and cross-linking of the butyl methacrylate and dimethacrylate monomers. Other peroxide catalysts which may be used are methyl ethyl ketone peroxide, cumene hydroperoxide, tertiary butyl hydroperox ide or any suitable source of free radical initiation for polymerization already known to the art.

Alternate plastic compositions for binding the potassium salts onto the rod may be used, such as polyethylene, cellulose acetate sorbate, polyvinyl acetate and/ or chloride, rubber or any equivalent, compatible binder which will give good adhesion to the holder, will not absorb functionally excessive amounts of nitroglycerin, and will not crack severely during temperature cycling of the device. Excessive absorption of nitroglycerin vapor into the coating is undesirable because of the effect of this compound on physical properties of the coating and on the rate of erosion of coating into the flame. This latter effect must be controlled with reasonable care since it is desirable, in most cases, that the potassium salt generation continue throughout the burning of the rocket grain. lt is particularly desirable that the rate of erosion not be increased as a result of storage of the rocket. The ballistic modifier compositions of the invention were found operable by external application with the propellent compositions disclosed in my above referred to application, thus establishing their operability with nitroglycerin-nitroceilulose base propellants in general. They are equally operable with double base propellants having equivalent `basic ingredients, as well as with composite propellants. The percentage of combined nitrocellulose-nitroglycerine content of propellants with which the ballistic modifier is operative may vary over comparatively wide limits.

The ballistic modifier of this invention has proved highly effective with propellent formulations having percentage compositions within the ranges shown below.

Component: W eight percent Nitrocellulose 44-60 Nitroglycerin 28-47 Stabilizer 1-5 Non-explosive plasticizer 3-15 Ballistic modifier 2-5 Examples of stabilizers are, symmetrical diethyl diphenyl urea and Z-nitrodiphenylamine. Examples of non-explosive plasticizers are, diethyl phthalate, triacetin, dipropyl adipate, dimethyl sebacate and dibutyl phthalate. Examples of ballistic modifiers are, lead salicylate, lead acetyl salicylate and lead 2,4-dihydroxybenzoate and mixtures of these compounds, used either alone or in combination with a lead salt of an organic acid other than salicylic acid. Preferred secondary lead salts are lead 2-ethylhexoate and lead stearate. The above showing of percentage compositions illustrates the operativeness of the present ballistic modifier with propellants or gasproducing charges having a nitrocellulose-nitroglycerin base, or the equivalent thereof. It is likewise operative with composite propellants having either of these compounds or their equivalents as the basic ingredient.

A modification of the invention is illustrated by the following example:

EXAMPLE II Benzoyl peroxide .15

lods having a coating of the above composition 0.30 inch in diameter and 9 inches in length were tested in a 2.0 rocket with a propellent grain made from the N-S propellent composition. The following results were obtained:

Burning time Burning time Conditioned tiring temperature, F. without rod, with rod,

seconds seconds The ballistic modifier compositions of this invention are made as follows: The potassium sulfate is first mixed with powdered ethyl cellulose ake using butyl methacrylate monomer as a softening agent. Dibutyl phthalate, polyglycol dimethacrylate and the peroxide catalyst are then `added to the mixture. This order of addition is not critical, the requirement being that uniform distribution of the peroxide must be obtained.

The above composition is conveniently mixed in a Sigma-blade type mixer such as that used Afor mixing bread dough, rubber or ordinary powder. After a dough of homogeneous character is obtained by this mixing process, it is placed in an extrusion press of conventional design and extruded through a die having the desired diameter and containing a pin or stake. A tube is thus produced having the desired internal diameter. The tube thus produced is placed over the wire or rod support and the assembly enclosed in a mold which is tightened around the molding both to insure good bonding to the holder and to exclude oxygen or air which inhibits or tends to inhibit the polymerization of the methacrylate monomers used in the coating composition. Curing times for achieving a hard, tough coating are of the order of a few hours at F. or a few minutes at a temperature of approximately 200 F. The above procedute is particularly useful when an oxidizing salt is used and excessive heating such as that incident to injection molding must be avoided.

In application, the coated rod is suspended from the head end of the rocket motor so that the coated portion of the rod is approximately centrally located in the perforation of the rocket grain which may 'be an internal or external-internal burning type or other type of grain. For entirely external burning grains the rods may be located in the annular space between grain and rocket motor.

The eficiency of the ballistic modifier may be increased by introducing it near the head end of the rocket grain, thereby ensuring that most of the potassium salt introduced traverses the entire length of the grain. ln this Way, its effect is prolonged for a longer period of time than would be the case if it were introduced within the powder or introduced along the entire length of the rod. Also, the additional heating time enhances the effect of the salt.

If the grain design or arrangement permits, the potassium salt may be introduced into the flame reaction by the use of a selfasustaining type of potassium salt based pyrotechnic which burns slowly and independently of the main propellent grain. Alternatively, a perforated tube which is loaded with loose potassium sulfate -may be used, the potassium sulphate `being brought into the reaction by the turbulent gases of the flame.

The invention provides a means by which the beneficial ballistic effects of potassium salts may be utilized in jet Iactuated devices utilizing mesa or plateau type propellants. The invent-ion not only avoids the undesirable effects attendant to internal introduction of potassium salts but actually provides a more eflicient method of utilization of ballistic modifiers, as its use requires less modifier than that required by internal introduction to reduce the secondary ame a corresponding amount. In addition, the ballistic effect of potassium salts is enhanced to a greater degree by external introduction into the flame as compared to internal introduction of the salts. Further advantages stemming from the invention are the following: the invention provides an extension of range over which ballistics of rocket motors using mesa and plateau type propellants are insensitive to temperature changes. For example, the service propellant N-5, used alone has a temperature insensitivity over the range of approximately 10 F. to +130 F. Application of the present invention to this propellant results inv a temperature insensitivity iover a range of approximately A 40 to -|140 F. `and the temperature coeliicient orV change in developed pressure with change in temperature is decreased over the entire temperature range desired, namely 65 to +165 F. ln detail, the effect of the invention under conditions where the rocket is red iat very low temperatures, such as -65 F., is to improve the smoothness of burning and to increase the minimum operating pressure significantly, thereby shortening the burning time. For example, in the 2,"75 FEAR rocket the minimum pressure at 65 F. is increased by approximately l or 20 percent and the burning time at this low temperature is reduced by percent. 'the internal ballistic etect `at +165 F. is to give smoother pressure-time traces by eliminating effects of resonance aud standing waves in the rocket motor. It has further been found that suppression of secondary flash or ame through the use of the invention is substantially complete, particularly during the early phases of burning, When the rocket gases can affect aircraft engine performance, and that the eciency of the potassium salt used to suppress flash is greater than that obtained if the potassium sulfate is introduced within the solid propellent composition itself. As a comparative example, the 2."75 FFAR Mk 1 motor employing the temperature-sensitive propellant known as Navy N-4 propellant was experimentally compared with propellants such as N5 which is relatively temperature insensitive. Without the use of the invention a far greater flash was produced in the case of N-5 propellant. When the invention was applied to N-S propellant the flash was noticeably less than `that produced by N- propellant. This latter propellant contains 1.5 percent `of potassium sulfate internally incorporated. The amount of potassium sulfate used on the coated rod for the N-S propellant experiment was appreciably less than 1.5 percent of the weight of the rocket grain.

Obviously, many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is: Y v

1. A jet actuated device comprising, in combination, a motor chamber having yat least one exit nozzle, ka smokeless double base propellent grain in said motor chamber, a ballistic modifier from the class consisting of potassium sulphate, barium nitrate, and titanium dioxide supported within the flame area ot said propellent grain, whereby the ballistic modilier is progressively eroded into the flame of the propellant when it burns and thereby enhances the ballstic effect of the propellant.

2. The device of claim 1 in which the ballistic modifier is potassium sulphate.

3. The device of claim l in which the ballistic modiiier is barium nitrate.

4. The device of claim 1 in which the ballistic modifier is titanium dioxide.

5. The device of claim 1 in which potassium perchlorate is added to the ballistic modifier.

6. The device of claim 1 in which the ballistic modifier is supported within the flame area of the propellent grain by means of a rigid member attached to the jet actuated device.

References Cited in the file o this patent UNITED STATES PATENTS 1,896,040 Ruben Jan. 3l, 1933 1,918,317 Benner et al July 18, 1933 2,050,871 Woodbridge Aug. 11, 1936 2,655,694 Piper Oct. 20, 1953 2,697,325 Spauldingr Dec. 2.1, 1954 OTHER REFERENCES v Gregory: Uses and Applications of Chemicals and Related Materials, Reinhold Publishing Corp., N.Y. (1939), 4th printing, vol. 1, page 485. 

1. A JET ACTUATED DEVICE COMPRISING IN COMBINATION, A MOTOR CHAMBER HAVING AT LEAST ONE EXIT NOZZLE, A SMOKELESS DOUBLE BASE PROPELLANT GRAIN IN SAID MOTOR CHAMBER, A BALLISTIC MODIFIER FROM THE CLASS CONSISTING OF POTASSIUM SULPHATE, BARIUM NITRATE, AND TITANIUM DIOXIDE SUPPORTED WITHIN THE FLAME AREA OF SAID PROPELLENT GRAIN, WHEREBY THE BALLISTIC MODIFIER IS PROGRESSIVELY ERODED INTO THE FLAME OF THE PROPELLANT WHEN IT BURNS AND THEREBY ENHANCES THE BALLSTIC EFFECT OF THE PROPELLANT. 